had to be some animal that’s abundant and relatively sedentary. A rodent? A small bird? A nonmigrating bat?
The evidence on each side of this dichotomy is varied and intriguing, though inconclusive. One form of that evidence is the genetic differences among variants of Ebola virus as they have been found, or left traces of themselves, in human victims, gorillas, and other animals sampled at different times and places. Ebola virus in general seems to mutate at a rate comparable to other RNA viruses (which means relatively quickly), and the amount of variation detectable between one strain of Ebola virus and another can be a very important clue about their origins in space and time. Peter Walsh, working with two coauthors on a paper published in 2005, combined such genetic data with geographical analysis to suggest that all known variants of Ebola virus descended from an ancestor closely resembling the Yambuku virus of 1976.
Walsh’s collaborators were Leslie Real, a highly respected disease ecologist and theoretician at Emory University, and a bright younger colleague named Roman Biek. Together they presented maps, graphs, and family trees illustrating strong correlations among three kinds of distance: distance in miles from Yambuku, distance in time from that 1976 event, and distance in genetic differences from the Yambuku-like common ancestor. “Taken together, our results clearly point to the conclusion that [Ebola virus] has gradually spread across central Africa from an origin near Yambuku in the mid-1970s,” they wrote. Their headline, stating the thesis plainly, was WAVE-LIKE SPREAD OF EBOLA ZAIRE. It may or may not be a new pathogen—at least, new in these places. (Other evidence, published more recently, suggests that filoviruses may be millions of years old.) But maybe something happened, and happened rather recently, to reshape the virus and unleash it upon humans and apes. “Under this scenario, the distinct phylogenetic tree structure, the strong correlation between outbreak date and distance from Yambuku, and the correlation between genetic and geographic distances can be interpreted as the outcome of a consistently moving wave of [Ebola virus] infection.” One consequence of the moving wave, they argued, is massive mortality among the apes. Some regional populations have been virtually exterminated—such as the gorillas of the Minkébé forest, of the Lossi sanctuary, of the area around Moba Bai—because Ebola hit them like a tsunami.
So much for the wave hypothesis. The particle hypothesis embraces much of the same data, construed differently, to arrive at a vision of independent spillovers, not a traveling wave. Eric Leroy’s group also collected more data, including samples of muscle and bone from gorillas, chimps, and duikers found dead near human outbreak sites. In some of the carcasses (especially the gorillas), they detected evidence of Ebola virus infection, with small but significant genetic differences in the virus among individual animals. Likewise they looked at a number of human samples, from the outbreaks in Gabon and the Congo during 2001–2003, and identified eight different viral variants. (These were lesser degrees of difference than the gaps among the five ebolaviruses.) Such distinct viruses, they proposed, should be understood in the context that their genetic character is relatively stable. The differences among variants suggest long isolation in separate locales, not a rolling wave of newly arrived, rather uniform virus. “Thus, Ebola outbreaks probably do not occur as a single outbreak spreading throughout the Congo basin as others have proposed,” Leroy’s team wrote, alluding pointedly to Walsh’s hypothesis, “but are due to multiple episodic infection of great apes from the reservoir.”
This apparent contradiction between Leroy’s particle hypothesis and Walsh’s wave hypothesis reflects an argument at cross-purposes, I think. The confusion may have arisen from back-channel communications and a certain sense of competition as much as from ambiguity in their published papers. What Walsh suggested—to recapitulate in simplest form—is a wave of Ebola virus sweeping across Central Africa by newly infecting some reservoir host or hosts. From its recent establishment in the host, according to Walsh, the virus spilled over, here and there, into ape and human populations. The result of that process is manifest as a sequence of human outbreaks coinciding with clusters of dead chimps and gorillas—almost as though the virus were sweeping through ape populations across Central Africa. Walsh insisted during our Libreville chat, though, that he had never proposed a continental wave of dying gorillas, one group infecting another. His wave of Ebola, he explained, has been traveling mainly through the reservoir populations, not through the apes. Ape deaths have been numerous and widespread, yes, and to